Modern wind turbines are commonly used to supply electricity into the electrical grid. Wind turbines of this kind generally comprise a rotor with a rotor hub and a plurality of blades. The rotor is set into rotation under the influence of the wind on the blades. The rotation of the rotor shaft either directly drives the generator rotor (“directly driven”) or through the use of a gearbox.
A generator comprises a generator rotor and a generator stator, wherein depending on the type of generator, the rotor and/or the stator may comprise a plurality of windings. The windings may be generally formed by a coil, or a plurality of concentric coils in series, electrically connected at its ends to e.g. a converter and/or another winding of the same phase.
A coil may be formed by a stack of turns of a conductor, e.g. a copper wire or sheet, around a core. Each of the turns may be electrically insulated from the other turns and may be formed by two substantially straight parallel segments, and two curved (corner) segments. Each of the turns continues into the turn directly on top of it or directly underneath it. The electrical insulation between the turns may be provided by e.g. insulating tape or varnish.
In this disclosure, a winding is regarded as a completed coil assembly (stack of turns of a conductor) with taps (connection points).
When one or more windings are broken or need to be replaced, it is common practice to remove the rotor (or the stator) from the generator, and replace the windings that need to be replaced. However, e.g. in direct drive wind turbines employing relatively large generators, it may be very difficult and a very cumbersome task to remove the rotor from the stator for maintenance works, if at all possible. In offshore wind turbines, this type of maintenance may even be more complicated.
An alternative method of repair is possible, if the stator is built up from various stator sectors, such as disclosed in e.g. US 2007/284959. Substitution of a coil in this case comprises substitution of a complete sector, including the stator frame, stator core, and coils. An important drawback of this method is that a part of the generator is substituted. Thus, subsequently a very precise adjustment of the air gap needs to be carried out on site.
There is thus still a need to provide a generator for wind turbines, and for other applications, in which windings may more easily be repaired. The methods and kits discussed in the present disclosure may equally be applied to any other kind of generator comprising windings mounted around teeth of either the generator or rotor. They may in particular be advantageous in applications with relatively large generators and/or in cases in which the access to the generator is complicated.